... avoid using bignum.

That's the rule that I live by.

To be fair, however, bignum is quite possibly easier to use than I think.
One just has to be aware of all the caveats ;-)

UPDATE:
i.e. object of the same class, with the same properties, same global parameters, but result is different.

I think the difference occurs because the "bignum" division is done by Math::BigFloat::bdiv, whereas the "bigint" division is done by Math::BigInt::bdiv.

Cheers,
Rob